Facilitating renewal of oil flow in a pipeline

ABSTRACT

Renewal of oil flow through a section of pipeline in which quiescent oil has gelled is facilitated by causing a segment of the gelled oil to yield in either an upstream or downstream direction. A portion of the yielded segment is removed from the pipeline. Liquid oil of relatively low resistance to flow as compared to the yielded segment is passed through the section of the pipeline previously occupied by the yielded segment. Thereafter, the removed oil is returned into the pipeline. The yielded segment may be removed at an elevation below a significant portion of the segment of the gelled oil to be yielded. If the elevation difference is in a downwstream direction, removal of the yielded segment may be used to decrease the hydrostatic pressure resisting renewal of oil flow. The hydrostatic head pressure caused by this elevation difference may be used to yield the segment of gelled oil and to cause a portion of the yielded segment to flow from the pipeline. As a less desirable alternative, a displacing fluid may be used to cause the segment of gelled oil to yield and flow from the pipeline.

United States Patent 91 Perkins FACILITATING RENEWAL OF OIL FLOW IN APIPELINE Thomas K. Perkins, Dallas, Tex.

Atlantic Richfield Company, New Y crk, N.Y.

Filed: Dec. 4, 1972 Appl. No.: 312,050

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 11/1972 Brown 137/236 X ABSTRACTRenewal of oil flow through a section of pipeline in which quiescent oilhas gelled is facilitated by causing a segment of the gelled oil toyield in either an upstream or downstream direction. A portion of theyielded segment is removed from the pipeline. Liquid oil of relativelylow resistance to flow as compared to the yielded segment is passedthrough the section of the pipeline previously occupied by the yieldedsegment. Thereafter, the removed oil is returned into the pipeline. Theyielded segment may be removed at an elevation below a significantportion of the segment of the gelled oil to be yielded. If the elevationdifference is in a downstream direction removal of the yielded segmentmay be used to decrease the hydrostatic pressure resisting renewal ofoil flow. The hydrostatic head pressure caused by this elevationdifference may be used to yield the segment of gelled oil and to cause aportion of the yielded segment to flow from the pipeline. As a lessdesirable alternative, a displacing fluid may be used to cause thesegment of gelled oil to yield and flow from the pipeline.

7 Claims, 1 Drawing Figure PATENIED UECZ 5l975 FACILITATING RENEWAL OFOIL FLOW IN A PIPELINE BACKGROUND OF THE INVENTION This inventionrelates to a method for facilitating renewal of flow of oil through apipeline containing gelled oil.

In cold regions where for long periods the prevailing ambienttemperature is cold enough to cool static oil to a temperatureapproaching or below its pour point, cessation of oil flow through anoil gathering line or feeder line, or trunk line will cause stationaryoil in the pipeline to cool and gel. This gives rise to the problem ofrestarting oil flow when the pipeline is shutdown for a period longenough for the oil to gel. This problem is encountered in the pipeliningof crude oils in cold climates like the arctic regions and in thepipelining of certain fuels or synthetic high pourpoint oils. When oilflow through agelled section of a pipeline is to be renewed, the statichead pressure, the length of the gelled section, and the yield strengthof the gelled oil may be such that it would require an undesirably highpressure to restore the flow of liquid oil through the pipeline. Therenewal pressure should not exceed the design pressure of the pipeline,the pumping stations, and related equipment. The desired renewalpressure for a system varies with the size of the line and the designconditions as well as various operating limitations, safety valves andequipment installed with the line.

Pipelines are laid up and down slopes, valleys, river beds, andmountains. This results in elevation differences between sections of thepipeline. For example, in one proposed oil pipeline for Alaska near theYukon River a hydrostatic head or gravity pressure in excess of 600 psicould occur. When oil flow through the pipeline is to be renewed, thishydrostatic pressure can severely limit the amount of additionalpressure that can be applied to the line. In addition, if the statichead pressure arises from a downstream direction, the hydrostaticpressure resists renewal of oil flow.

When oil flow through a pipeline is to be renewed, oil is pumped intothe pipeline until the yield pressure is reached. Under certainconditions, especially where downstream sections of the pipeline areexperiencing high hydrostatic pressures, initial pump injection rateswill be low and it may take long periods of time to restore the desiredoil flow rate. Yet, there is a minimum flow rate compatible with theinstalled pumps that must be maintained. Otherwise, the pump temperaturestarts to rise rapidly burning out seals and bearings and the likeunless heat is removed or the pump is stopped by temperature safetydevices. The minimum desirable flow rate can be quite large and on theorder of tens of thousands of barrels of oil per day.

Even if other startup systems are used, there is a maximum allowed timefor restoring and bringing oil flow back up to the desired level. Downtime is costly and creates collateral problems. The startup'systemshould be reliable and the time required to restore flow should bereasonable and reliably predictable.

SUMMARY OF THE INVENTION Renewal of flow of liquid oil through apipeline in which stationary oil has formed a stationarygel isfacilitated in a way which reduces the startup pressure. The method offacilitating renewal of oil flow permits startup at a flow rate highenough to avoid overheating ofpumps installed to pump and maintain therate of oil flow through the pipeline. The method is useful for reducingthe startup time of a pipeline in a reliable and predictable manner. Themethod is especially suited for a pipeline which is laid on and followsthe contour of uphill and downhill terrain where hydrostatic head orgravity pressure limits the amount of pressure increase that can beapplied to startup a pipeline. The method for facilitating renewal ofoil flow through a gelled oil section of a pipeline is especiallyadvantageous for restoring flow through a section where renewal of flowis resisted by hydrostatic head pressure caused by downstream elevationdifferences.

The normal startup pressure through a section containing gelled oildepends on the yield strength of the gelled oil, the internalpipediameter, the length of the gelled oil section, and the hydrostatic headpressure.

The length of the gelled oil section is shortened and the normal startuppressure is reduced by causing a segment of the gelled oil to yield andby removing a substantial portion of the yielded segment from thepipeline at a point between the'inlet and the outlet of the pipeline.Thereafter liquid oil having a yield strength and flow resistancesubstantially less than the yielded segment is flowed through thesection of thepipeline from whence the yielded segment came. The removedyielded gelled oil canthen be returned to the pipeline into liquid oilflowing therein.

Reducing the normal startup pressure permits restoration of oil flow atastartup flow rate compatible with the pipeline pumps thereby lesseningthe chances of overheating the pumps.

Preferably, a significant portion of the yielded segment of gelled oilcomes from an elevation higher than the point of removal and thehydrostatic head pressure created by the elevation difference caused thesegment to yield and flow from the pipeline. When the removal point isupstream from the yielded segment, the yielded segment flows upstreamand reduces the hydrostatic head pressure resisting oil flow in thedownstream direction.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a fragmented view of asection of a pipeline illustrating a preferred way of facilitatingrenewal of oil flow through a gelled section of the line.

DETAILED DESCRIPTION OF THE INVENTION Oil flowing in a pipeline atnormal temperatures behaves as a Newtonian fluid. When a pipeline whichpasses through a region where the ambient temperature is below the pourpoint of the oil is shutin, the static oil cools increasing in yieldstrength and gels into a thixotropic material with a relatively highresistance to flow. This quiescent gelled oil hampers or blocks renewalof oil flow from an upstream point to a downstream point.

When oil flow is to be renewed, the total startup pressure required tocommence movement of the gelled stationary oil is dependent upon theyield properties of the gelled oil, the length of the gelled oilsections, the hydrostatic head pressures resisting restoration of oilflow, and, to a point, on the rate of application of the pressure. Themaximum startup pressure differential that can be applied is the maximumdesign pressure of the pumping stations or the safe operating pressureof the pipeline and related equipment, whichever is less, minus thehydrostatic pressure. If the permitted pressure differential isrelatively small or the startup pressure or the hydrostatic pressure isunduly high, the startup pump injection rates could be low enough tooverheat the pumps and/or low enough to require an excessive period oftime to restore the desired flow rate and at the same time causeoverheating of the pumps. As a result, it is important that the totalstartup pressure not only be below a pressure which will damage thepipeline and related equipment but also below a pressure which willundesirably reduce the pump injection rate and unduly lengthen the timeto restore the desired flow rate. 1

In this invention, liquid oil flow from an upstream point or atoward-the-source point through a gelled section or sections of thepipeline to a downstream point or an away-from-the-source point isfacilitated in a reliably predictable manner which alleviates theaforementioned difficulties. in one embodiment, the need for sideequipment is minimized, and in another embodiment, downstreamhydrostatic pressures resisting renewal of oil flow are reduced.

As shown in the drawing, pipeline 11 is laid on uphill and downhillterrain l3 and generally follows the contour of the terrain. Thepipeline may be laid on the ground or buried, or only partially orintermittently buried with some sections laid on the surface or elevatedon gravel berms or suspended above the ground on pile bents. Prior tothis invention, the section pipeline illustrated was blocked by gelledstationary oil. Flow of oil from an upstream point (not shown) throughthe illustrated section to a downstream point (not shown) is scheduledfor renewal. At an appropriate point intermediate or between theupstream point and the downstream point is removal point 15 formed by aside drain line which acts as a specially-located removal pointpermitting removal or drainage of a substantial segment of the gelledoil as hereinafter set forth. The drain line leads to an appropriatestorage facility 17 which has a capacity large enough to a preselectedamount of partially-gelled oil as'hereinafter set forth. Connected tothe drain line or storage facility 17 is pump or injection means 19adapted to return the partially-gelled oil from the storage facilityinto pipeline I]. The injection means or pump 19 may be relatively smallsince the rate of return of the partiallygelled oil 'to the pipeline hasnothing to do with the method for facilitating renewal of oil flow inthe pipeline.

Preferably as illustrated, removal point 15 created by the drain line isat a net elevation substantially lower than at least a portion of thepipeline to be drained. Under these conditions, the elevation differencecauses the oil column to create a hydrostatic head pressure on theremoval point. in one preferred embodiment of the 1 method of thisinvention, this hdyrostatic head pressure is at least as great as theyield strength of the segment of gelled oil to .be drained at theremoval pointand isextends from removal point 15 to an vupstream or adownstream point. A segment of gelled oil between two points willexhibit a certain yield strength. The pressure differential across thelength of a segment of gelled oil can be increased until the gelled oilsegment begins to visibly move. The pressure differential at which agelled oil segment visibly starts to flow after a static period at thattemperature is therein called the yield pressure. The force pushing thegel is approximately the product of the internal area of the pipelinemultiplied by the pressure differential (AP), and at the point of yield,this force is equal to the shear resistance of the gelled oil segment.The shear resistance of the gelled oil segment is approximately equal tothe product of the apparent yield strength or shear stress (1')multiplied by the internal surface area of the pipeline for the length(L) of the segment chosen, that is vrDL where D is the internal diameterof the pipeline. As used herein, the apparent shear stress incorporatesallowance for visible movement of the gel and a reasonable rate ofapplication of the startup pressure. The following Equation 1 is anexpression of the equality of force and shear resistance at the yieldpressure:

( 1) Equation 1 can be simplified into the following Equation 2:

Equation 2 is useful for roughly estimating the yield pressure betweentwo points and for plotting scaling or comparing experimental data. Formost pipelines, the ratio of 4L to D is large for the section of thepipeline between pumping stations. The distance between pumping stationsdepends on a number of factors including size of the pipeline, the flowrate, the design operating pressure, topographical and rheologicalfactors, and economics. The conditions affecting the yield strength andthe yield pressure of a segment of gelled oil will be hereinafterdiscussed.

The gelled oil is thixotropic and shear rate degradable. Once the yieldpressure is exceeded and the gelled oil begins to move, sheardegradation takes place where there is a velocity gradient betweenlayers of the gel. This shear degradation significantly reduces theyield pressure of the degraded gelled segment by about 40 to 60 percent. The rate of shear degradation depends on the rheologicalproperties of the oil and its gel characteristics, and on the rate ofmovement of the segment. It has been found that the rate of sheardegradation initially undergoes a rapid degradation. Thereafter, therate of shear degradation tapers off to a relatively gradual rate. Thisreduction in yield pressure of the yielded segment is very useful forremoving a portion of the yielded segment from the pipeline at removalpoint 15.

A portion of the yielded segment is removed from the pipeline at removalpoint 15 through the drain line to storage facility 17. Further sheardegradation will usually occur as the yielded segment passes through thedrain line. Normally additional degradation will not be required. Thedegraded gelled oil will not rebuild its gel strength for a period longenough for the method of this invention to be carried out. Nevertheless,if it is desirable to further degrade the yielded segment or to maintainits flow properties, the storage facility mayv be heated or the oil maybe circulated and degraded by injection means 19.

Yielding and removal of a portion of the gelled oil segment facilitatesrenewal of oil flow through the pipeline in two ways. First, aspreviously mentioned, yielding of the gelled oil segment shear degradesthe yielded gel thereby significantly decreasing its yield strength.Thus, the part of the yielded segment, if any, left in the pipeline hasa significantly lower flow resistance than the previously quiescent gel.However, as compared to liquid oil, the yielded segment still has arelatively high flow resistance. Secondly, removal of a portion orsubstantially all of the yielded gelled oil segment reduces the yieldpressure of the remaining unyielded gelled oil in the pipeline. Aspreviously shown, the yield pressure is proportional to length. Removalof the yielded segment shortens the length and thereby reduces the yieldpressure. Removal of the yielded segment also lessens the total flowresistance because the yielded segment still exhibits a relatively highflow resistance as compared to liquid oil. Removal of a portion or allof the yielded segment has a still further advantage as will hereinafterbe shown where such removal reduces the hydrostatic head pressureresisting renewal of oil flow.

The segment of gelled oil can be caused to yield in several ways. Afluid may be injected into pipeline 1] by way of any means suitable forintroducing a fluid into a pipeline to cause the segment to yield and toflow from the pipeline by way of removal point 15. The max imuminjection pressure and rate of fluid injection will, of course, berestricted and controlled so that the injection pressure will remainbelow a pressure which will damage the pipeline and related equipment.

More preferably, when the pipeline is being designed, the location ofremoval point is selected in a manner such that a portion of the segmentof gelled oil to be yielded is at an elevation higher than the removalpoint and the segment may be caused to yield and to be removed in anupstream direction. In this situation, the elevation difference createsa hydrostatic head pressure resisting renewal of oil flow through thegelled segment. The gelled segment is caused to yield toward the removalpoint and to be removed from this downstream direction. As a result,removal of the yielded segment not only removes relatively high flowresistant partiallydegraded gelled oil, but also decreases thehydrostatic head pressure resisting renewal of oil flow.

In the most preferred embodiment of this invention, the selected sitefor the removal point is not only upstream from and at an elevationlower than a portion of the segment of gelled oil to be yielded, but isalso at a point where the net hydrostatic head pressure caused by theelevation difference is at least as great as the yield strength andyield pressure of this segment. This hydrostatic head pressure is usedto cause the segment of gelled oil to yield in an upstream directiontoward the removal point and to cause at least a portion of the yieldedsegment to flow from the pipeline through removal point 15 to storagefacility 17. As previously mentioned, when the segment yields, sheardegradation occurs and the resistance to flow is quickly andsignificantly reduced by about 40 to 60 per cent of the yield pressure.As a result, no special injection equipment is needed to cause thesegment to yield and to cause all or a portion of the. yielded segmentto flow from the pipeline. The yielded segment will continue to flow aslong as the net decreasing hydrostatic pressure is sufficient toovercome the decreasing flow resistance of the yielded segment.

Yielding and removal of at least a portion of the segment of gelled oil,therefore, facilitates renewal of liquid oil flow from a point upstreamof the removal point through the section of the pipeline from whence theyielded segment came to a point downstream of the removal point at apressure below the startup pressure which would normally have beenrequired to displace oil through this section. Accordingly, liquid oilflow commenced pressure and oil is passed from the upstream pointthrough the section of the pipeline from whence the yielded segmentcame. The liquid oil has a flow resistance much lower than the gelledoil. Flow of liquid oil quickly restores the flow capacity of thissection of the pipeline. Thereafter the partially degraded yielded gelremoved from the pipeline is returned by injection means 19 to thepipeline at any point where liquid oil is flowing therein. The rate ofreturn of this previously removed oil is practically immaterial to theprocess and the rate of return can be quite small.

The following conditions affect the yield strength and yield pressure ofa segment of gelled oil. Crude oils are usually a complex mixture ofoils, waxes, asphalts, bitumens, and resins with a wide range of meltingpoints, cloud points, or pour points. Some'of the effects observed wheninvestigating the factors influencing the gel strength and yieldpressure of an oil are complex and not fully understood. Concepts orexplanations have been proposed which explain at least in part what hasbeen observed and it has been fully demonstrated that the yield pressureof an oil developed during period of quiescence varies widely with thecomposition of the oil, the temperature, the thermal history of the oil,the rate of cooling, the previous shear history of the oil, aging of theoil, and the compressiblity of the system.

The effects of asphalt, waxes, and the like components on the pour pointof oil is well known. In addition, the gel strength of an oil issensitive to the presence or absence of lightends. Weathering, flashing,or removal of light ends leads to a significant increase in gelstrength. Moreover, flashing of live crude oil from a producing well toatmospheric pressure in one step leads to a higher yield strength thanmultistep flashing. Of course, the presence of large percentages ofwater would affect yield, but water is usually separated from the oilbefore the oil is passed through the pipeline. The mixing of even asmall quantity of high yield strength oil tends to lead to a mixturehaving the yield strength like the high yield strength oil.

In general, decreasing the gel temperature and increasing the rate ofcooling increase the yield strength and in turn cause a higher yieldpressure.

As to shear history, it has been found that subjecting the oil to highshear conditions tends to increase the yield strength of the gelled oilafter it is subsequently cooled to a low temperature. The observedeffect of shear depends in part on the temperature of the oil during thetime of shear application. The temperature in turn is affected by thecomposition of the oil.

Aging and thermal history are in some respects alike. As used herein,however, aging relates to time at a given temperature. In general, for aperiod of time the gel strength of a gel increases. Results indicatethat the gel strength begins to stabilize in about 8 to 10 hours ofresidence time.

The thermal history of an oil primarily relates to cycles in temperaturewith the peaks in high and low temperatures encountered being important.Generally, if 5 an oil is returned to a high temperature and held atthat temperature for a period of time, the effects of piror thermalhistory are for practical purposes erased. The temperature to which theoil must be raised depends on the composition of the oil. If thetemperature of the oil is cycled in a manner such that the oil neverreaches the restoration temperature, cycling tends to cause an increasein yield pressure when the oil is subsequently cooled and gelled. Theeffects of cycling on, yeild pressure depend in part on the temperatureto which the oil is raised during a cycle. The effects of a peaktemperature during a cycle depend on the composition of the oil. Ingeneral, for normal crude oil, it has been found that cycling of thecrude oil at a temperature below 40 F increases the ultimate yieldpressure of the oil when the oil is cooled and gelled. The mechanism bywhich the yield increase occurs can be explained in the followingmanner. The asphalt micelles and paraffin'crystals form simultaneouslywhen the oil is cooled. If the oil is cooled to a temperature aboveitsgel point or pour point, the cooled oil has no structure andconsequently very little gel strength. Upon reheating of the cooled oil,the paraffin tends to go back into solution while the asphalt micellesare not significantly affected unless the temperature is raisedsignificantly. When the temperature is again lowered, the paraffincrystals reform but this time they form within and around the asphaltmicelles. This results in a paraffin-asphalt gel structure ofconsiderable strength at low temperatures; consequently, the ultimateyield pressure of a gelled oil which has been subjected to this type ofcycling is quite high.

Equations have been developed and published which for purpose of thisinvention adequately predict yield behavior of gelled, thixotropic oilsin pipelines. in general, the shear strength ofa certain oil at acertain temperature is approximately a point function of shear rate andshear strain. The relationships involved have been verifiedexperimentally by first returning the oil to its original condition.This is best accomplished by reconstitution of the oil, if desired, andby reheating the oil to some temperature level, for example, thereservoir temperature or some other preceding high processtemperature-This temperature level is held for a period of time toovercome the effects of prior yield afiecting factors. Thereafter,subsequent factors like rate of cooling, shearing. mixing, temperaturecycling, and the like, which measurably influence yield strength, can becontrolled and varied or simulated to develop the desired correlations.

EXAMPLE- A 48-inch OD crude oil pipeline for Prudhoe Bay, Alaska, havinga wall thickness of 0.462 inch and an internal design pressure of 832psi has a 70-mile section between two pumping stations starting at ariverbed at an elevation of about 300 feet which is a low upstream pointfor this section of the pipeline. The pipeline is made up of altemating5-mile increments of buried and surface line and is laid up and downterrain with the 70- mile downstream point being at an elevation of2,000 feet. This section of the pipeline is filled with oil and thehydrostatic head pressure at the riverbed is 650 psi.

The pipeline has been shut-in long enough for the oil to gel, but notlong enough for the temperature of the buried increments to fall below32 F. The gel in the surface increments is 20 F. In this example, thePrudhoe Bay crude oil has a yield strength of 0.1 dyne per squarecentimeter at 32 F. At 20 F, this crude oil exhibited a yield strengthbetween 15 and 900 dynes per square centimeter depending on beneficiatedand nonbeneficiated history or treatment. For sake of illustration, itis first assumed that the yield strength at 20 F is dynes per squarecentimeter and that it is desired that the pump startup pressure notdrop below 500,000 barrels of oil per day (BOPD). Under the illus tratedconditions with a maximum pressure not to exceed 832 psi, the pump ratefalls below 500,000 BOPD in about.4.5 minutes and it takes about 56minutes for the startup flow rate to rise again to the 500,000 BOPDlevel. However, if the hydrostatic head pressure is used to yield theoil toward the riverbed and 13,000 barrels of oil are drained attheriverbed in accordance with the method of this invention, an estimatedstartup flow rate of 500,000 BOPDmay be maintained. If it is assumedthat the yield strength at 20 F is and 200 dynes per square centimeter,used of the hydrostatic head pressure to yield the oil toward theriverbed and drainage of 160,000 barrels and 240,000 barrels,respectively, will allow maintenance of the 500,000 BOPD startup flow,rate. This illustrates the advantages of the method of this inventionfor facilitating renewal of oil flow through a gelled section of apipeline.

The foregoing description illustrates preferred embodiments of a methodfor facilitating renewal of oil flow in a pipeline. Reasonablevariations and modifications are practical within the scope of thisdisclosure without departing from the spirit and scope of the claims ofthis invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A method for facilitating renewal of flow of oil from an upstreampoint to a downstream point through a pipeline which generally followsthe contour of uphill and downhill terrain and in which oil has formed astationary gel, which method comprises causing at least one segment ofthe gelled oil to yield toward a removal point between said upstream anddownstream points, removing a portion of the yielded segment from thepipeline at said removal point, passing liquid oil from said upstreampoint through the section of said pipeline from whence said yieldedsegment came, and thereafter returning the removed oil to said pipelineinto liquid oil flowing therein.

2. The method according to claim 1 wherein at least a portion of atleast one segment of. gelled oil that is caused to be yielded is at anelevation higher than said removal point.

3. The method according to claim 1 wherein at least one segment of thegelled oil is caused to yield in an upstream direction toward saidremoval point and at least a portion of said segment is at an elevationhigher than said removal point. 1

4. The method according to claim 1 wherein one segment of the gelled oilis caused to yield in an upstream direction toward the removal point anda portion of said one segment is removed from said pipeline at saidremoval point, another segment of the gelled oil is caused to yield in adownstream direction toward said removal point and a portion of saidanother segment is removed from said pipeline at said removal point, andliquid oil is passed from said upstream point through the section ofsaid pipeline from whence both of said yielded segments came.

5. The method according to claim 1 wherein at least a portion of atleast one segment of the gelled oil to be yielded is at an elevationhigher than the removal point and the hydrostatic head pressure causedby said elevation is at least as great as the yield pressure of saidsegment, and said hydrostatic head pressure is used to cause saidsegment to yield and to cause at least a portion of said yielded segmentto flow from said pipeline at said removal point.

6. The method according to claim 5 wherein said segment is caused toyield in an upstream direction.

7. The method according to claim 1 wherein at least a portion of onesegment of the gelled oil to be yielded is at an elevation higher thanthe removal point and the hydrostatic head pressure caused by saidelevation is at least as great as the yield pressure of said onesegment, and said hydrostatic head pressure is used to cause said onesegment to yield in an upstream direction toward said removal point andto cause a portion of said yielded one segment to flow from saidpipeline at said removal point, at least a portion of another segment ofthe gelled oil to be yielded is at an elevation higher than said removalpoint and the hydrostatic head pressure caused by said elevation is atleast as great as the yield pressure of said another segment, and saidhydrostatic head pressure is used to cause said another segment to yieldin a downstream direction toward said removal point and to cause aportion of said yielded another segment to flow from said pipeline atsaid removal point, and liquid oil is passed from the upstream pointthrough the section of said pipeline from whence both of said yieldedsegments came.

1. A method for facilitating renewal of flow of oil from an upstreampoint to a downstream point through a pipeline which generally followsthe contour of uphill and downhill terrain and in which oil has formed astationary gel, which method comprises causing at least one segment ofthe gelled oil to yield toward a removal point between said upstream anddownstream points, removing a portion of the yielded segment from thepipeline at said removal point, passing liquid oil from said upstreampoint through the section of said pipeline from whence said yieldedsegment came, and thereafter returning the removed oil to said pipelineinto liquid oil flowing therein.
 2. The method according to claim 1wherein at least a portion of at least one segment of gelled oil that iscaused to be yielded is at an elevation higher than said removal point.3. The method according to claim 1 wherein at least one segment of thegelled oil is caused to yield in an upstream direction toward saidremoval point and at least a portion of said segment is at an elevationhigher than said removal point.
 4. The method according to claim 1wherein one segment of the gelled oil is caused to yield in an upstreamdirection toward the removal point and a portion of said one segment isremoved from said pipeline at said removal point, another segment of thegelled oil is caused to yield in a downstream direction toward saidremoval point and a portion of said another segment is removed from saidpipeline at said removal point, and liquid oil is passed from saidupstream point through the section of said pipeline from whence both ofsaid yielded segments came.
 5. The method according to claim 1 whereinat least a portion of at least one segment of the gelled oil to beyielded is at an elevation higher than the removal point and thehydrostatic head pressure caused by said elevation is at least as greatas the yield pressure of said segment, and said hydrostatic headpressure is used to cause said segment to yield and to cause at least aportion of said yielded segment to flow from said pipeline at saidremoval point.
 6. The method according to claim 5 wherein said segmentis caused to yield in an upstream direction.
 7. The method according toclaim 1 wherein at least a portion of one segment of the gelled oil tobe yielded is at an elevation higher than the removal point and thehydrostatic head pressure caused by said elevation is at least as greatas the yield pressure of said one segment, and said hydrostatic headpressure is used to cause said one segment to yield in an upstreamdirection toward said removal point and to cause a portion of saidyielded one segment to flow from said pipeline at said removal point, atleast a portion of another segment of the gelled oil to be yielded is atan elevation higher than said removal point and the hydrostatic headpressure caused by said elevation is at least as great as the yieldpressure of said another segment, and said hydrostatic head pressure isused to cause said another segment to yield in a downstream directiontoward said removal point and to cause a portion of said yielded anothersegment to flow from said pipeline at said removal point, and liquid oilis passed from the upstream point through the section of said pipelinefrom whence both of said yielded segments came.